Budget : Setting a budget out front can be beneficial if you have budget constraints. Having a budget to work on, you can easily adjust the types of material to be used or length of wooden planks to be purchased accordingly before ordering the materials.
Islands can "float" as well. Smaller islands used primarily as an aid in cooking or baking can be fitted with wheels and moved around the kitchen as needed to aid in cooking. Whether you consider a floating island will probably depend on what you have installed on the floor now, or plan to install (a soft wood will be nicked and scratched).
Pick a countertop for your island that matches your other counters or make it stand out by using a contrasting material. Also, your choice of cabinetry and decorative elements can make your island into a real quality item, similar to a fine furniture piece.
Space and Design are Important : You need around 3 feet of walking space around the island unit, and you must be sure that the drawers and doors in the cabinets have room to open. If you keep to the 36 inches then you should be OK. The only dimension that must be stated is that of the counter height. Ideally it should be the same as the rest of your kitchen units. If not, then it is not a disaster, but can be disconcerting when you are working on the various surfaces at varying heights.
small kitchen islands with seating
small kitchen island with sink and dishwasher
small kitchen island with drop leaf